MILWAUKEE, Wis. - The joke was on Mitt Romney today when his staff played an April Fool's trick on him, directing him to speak to an empty room instead of the packed ballroom where his event was actually scheduled to be held, shocking him with a non-existent audience just days before the Wisconsin Republican presidential primary.

"This is a morning I'm not going to forget anytime soon," said Romney, who was introduced by Rep. Paul Ryan and Sen. Ron Johnson, who endorsed him earlier today.

"You know it's April Fool's Day. I forgot. They turned me into the April Fool this morning, those guys. Your senator, your congressman," Romney told the crowd that was gathered for a pancake brunch at the Bluemound Gardens event space. "Downstairs is a room just like this one, and it was all set up."

Romney said the black curtain that usually shields him from the crowd before he enters was set up, as was the tape on the floor instructing him and the introducers where to stand. By all accounts, Romney believed he was readying to address the breakfast crowd.

"They're all standing there. Paul Ryan goes out first, all right, and this is downstairs, there's not a person in the room, and my staff says to me, 'There's a small - It's a small - We didn't get much of a turnout this morning, it's really small, but it will be OK, it will be OK,'" Romney said. "And so I hear Paul Ryan goes out, gives the same introduction, what he just said. He's down there, 'And so now, let's welcome Ron Johnson, and Mitt Romney, the next president of the United States.'

"And so the two of us go out there and it's completely empty," Romney said. "There's nobody there. I thought, oh, boy, this is going to look really bad on the evening news, let me tell you."

Romney said the entire prank was caught on video by his staff members, who had orchestrated the joke the night before.

"You guys - this is known as forgive but remember," Romney said, laughing. "I'll tell you. We're going to remember this."

No word yet on what kind of revenge Romney, a known prankster, might launch on his staffers before the day is over.

An aide for Romney said that the campaign planned to post video of the prank online later today. In the video, shown to members of the press corps, Romney can be heard saying, "You guys are bad," after walking into the empty room.

Later in the day, stopping at a Culver's burger joint for food, Romney told the press that his first reaction when he saw the empty room was, "Oh my goodness, this is really a problem."

Asked if he has any plans for retaliation yet, Romney said he's still thinking about it, saying that it "wasn't fair," that he had been so gullible.

"So we've got to find a way to get back at Garrett, Charlie the whole crew," said Romney, naming some of his aides. "I [pranks] are much funnier when I do them on other people than when they do them on me. I'm far better at the ones I get to do on other people but this was very good. Very very good, this was classic."